True fasters will tell you that you are not doing the best to your body by taking diet pills. You won't do the full detox and your just adding checmicals to your diet instead of lessening them. If your goal, in whole or in part, is to rid yourself of toxins and clean out your body, taking diet pills isn't the best choice.
If you are fasting for the sole purpose of weight loss. The diet pills might help you get through the day, either with energy or with an appetite suppressant. And whatever helps you.
Can I make a suggestion though. Before you do the diet pills, try green tea. It has caffeine in it which gives energy, is an appetite suppressant and a diuretic. It helps me and it's a natural choice.
Let me know if you have any more questions.